The demand for hot melt glue binding, i.e. perfect binding, equipment, such as of the type shown in international patent applications WO 03/064169 A or U.S. Pat. No. 5,376,008 A of the present applicant, has increased considerably in recent times. This is partly a result of the rapid development of digital printing technology. Using this type of equipment it is possible to produce books in short runs, or even in individual copies, in a way that is economically reasonable.
A photo book is a typical example of a book that is produced in short runs or in individual copies. Due to the advance of digital camera and picture processing technology the demand for photo books in particular has grown a lot.
When printing is done using modern digital printing technology in which the paper passes between several rollers and cylinders, the best result is currently obtained by printing on a coated and/or strongly calendered paper grade. In this case, the print quality is excellent at best, and it is possible to reproduce colors very close to the desired result.
Experience has shown that perfect binding works best when a book is made of paper that is uncoated and non-calendered. However, it is not possible, using modern digital printing technology, to achieve a final print quality on this kind of paper that would match the quality achievable if the printing were done on a coated and/or strongly calendered paper grade.
Currently, and with respect to short runs, perfect binding is most commonly carried out by using an ethylene vinyl acetate (EVA) hot melt copolymer. The working temperature of EVA hot melt is about 170° C. and the glue has to be applied to the spine of the book in that is to be bound the so-called open time that is about 10-60 seconds during which the glue has not yet solidified and can be worked. EVA hot melt glue melts when reheated in a bookbinding device.
On an industrial scale, books are currently increasingly bound by using a polyurethane (PUR) melt glue. The working temperature of PUR glue is approximately 130° C. PUR glue solidifies as it cools, but continues to react with moisture in the air before it gains its ultimate strength. In this case, the glue is turned into an insoluble urea. Such a solidified PUR glue cannot be melted when reheated.
A binding made with EVA hot melt does not withstand heat as well as a binding made with PUR glue. With respect to certain paper grades a better binding strength is obtained with PUR glue than with EVA hot melt. The use of PUR glue is more difficult than the use of EVA hot melt, however, because the bookbinding devices that use PUR glue require intensive cleaning to ensure that the equipment can continue to be used.